Most wiring patterns of integrated circuits are formed of aluminum wiring lines, and there is a tendency for methods of forming a layer insulating film, such as a SiO.sub.2 film (silicon dioxide film) or a SiOF film (silicon oxide film containing fluorine), for insulating the aluminum wiring lines to employ an ECR (electron cyclotron resonance) plasma process because of the ability of the ECR plasma process to form films in a satisfactory quality.
Referring to FIG. 12 showing, by way of example, a conventional plasma processing system for carrying out the ECR plasma process, a 2.45 GHz microwave is propagated through a waveguide 11 into a plasma producing chamber 1A, a magnetic field of a predetermined magnetic field intensity, such as 875 G, is applied to the plasma producing chamber 1A by a solenoid 12 to produce a high-density plasma from a plasma producing gas 15, such as Ar gas and O.sub.2 gas, by the interaction (resonance) of the microwave and the magnetic field, a reactive gas 16, such as SiH.sub.4 gas or SiF.sub.4 gas, supplied to a film forming chamber 1B is activated and ionized by the plasma, and a thin film is deposited on the surface of a semiconductor wafer W placed on a wafer stage 13.
As shown by way of example in FIG. 13, the waveguide 11 is formed by connecting a conical waveguide section 11b to the lower end of a rectangular waveguide section 11a having a bend therein for transmitting a transverse electric wave in a TE.sub.11 mode (hereinafter referred to simply as "TE mode"). The lower end of the conical waveguide section 11b is joined to the upper end of a vessel defining the plasma producing chamber 1A. When a microwave is generated by a microwave generator 14 connected to the other end of the rectangular waveguide section 11a, the microwave is propagated in a TE mode in the waveguide 11 to the plasma producing chamber 1A. When the rectangular end of the bent rectangular waveguide section 11a is connected to the circular upper end of the conical waveguide section 11b, the microwave propagated in a TE mode in the rectangular waveguide section 11a is propagated also in a TE mode in the conical waveguide section 11b.
The propagation of the microwave in a TE mode in a cylindrical waveguide of an inside diameter 2a will be described with reference to FIGS. 14A and 14B. FIG. 14A is a cross-sectional view, and FIG. 14B is a sectional view taken on line A--A in FIG. 14A. In FIG. 14A, solid lines represent an electric field, and broken lines represent a magnetic field. In FIG. 14B, blank circles .smallcircle. indicate a direction of an electric field into the paper, and solid circles .circle-solid. indicate a direction of an electric field out of the paper. In a TE mode, the electric field is parallel to the diameter of the conical waveguide, and .lambda.=3.41a, where .lambda. is the wavelength of the microwave, and a is the radius of the cylindrical waveguide.
As is obvious from FIG. 14A, the density of lines of electric force is high in a central region and decreases toward a peripheral region if the microwave is guided in a TE mode into the plasma producing chamber 1A by the foregoing waveguide. Therefore the field intensity in the peripheral region is lower than that of the central region and hence the distribution of field intensity of the electric field is not uniform. Consequently, the density of a plasma produced by the agency of the microwave is low in the peripheral region and hence it is difficult to deposit a film on the surface of the wafer in a highly uniform thickness. Incidentally, the importance of the high uniformity of a film formed on the surface of a wafer has increased because pattern miniaturization has a tendency to advance in recent years.
A plasma processing system disclosed in U.S. Pat. No. 5,234,526 uses a TM.sub.01 mode. This plasma processing system employs a waveguide formed by connecting one end of a rectangular waveguide section for propagating a microwave in a TE mode to one side of a cylindrical waveguide section of, for example, 109 mm in inside diameter to propagate a microwave in a TM.sub.01 mode into a plasma producing chamber, and the lower end of the cylindrical waveguide section is joined to an upper end of a vessel defining the plasma producing chamber. In this waveguide, the joint of the rectangular waveguide section and the cylindrical waveguide section serves as a TM mode converter for converting wave propagation mode from a TE mode to a TM.sub.01 mode.
Wave propagation in a TM.sub.01 mode by the cylindrical waveguide of 2a in inside diameter will be described with reference to FIGS. 15A and 15B. FIG. 15A is a cross-sectional view, and FIG. 15B is a sectional view taken on line A--A in FIG. 15A. In a TM.sub.01 mode, lines representing an electric field extends from the wall of the waveguide through a central region of the waveguide and to the wall of the waveguide, and the electric field changes its direction every half the wavelength as it is propagated, and .lambda.=2.61a, where .lambda. is the wavelength of the microwave, and a is the inside radius of the cylindrical waveguide. In FIG. 15B, broken lines represented magnetic field, blank circles .smallcircle. indicate a direction of an electric field into the paper, and solid circles .circle-solid. indicate a direction of an electric field out of the paper.
When processing a wafer of, for example, 6 in. in diameter by this plasma processing system, an electric field of a uniform field intensity can be created because the inside diameter of the cylindrical waveguide is 109 mm. However, there is a tendency in recent years for the diameter of wafers to increase. If a wafer has a diameter greater than the inside diameter of the cylindrical waveguide, field intensity in a region of the exit of the waveguide corresponding to the peripheral region of the wafer is reduced. Consequently, the density of the plasma in a region near the circumference of the wafer becomes smaller than that of the plasma in a region corresponding to the central part of the wafer, so that the distribution of the density of the plasma on the surface of the wafer becomes irregular and a film having a not satisfactorily uniform thickness is formed on the surface of the wafer.
The present invention has been made in view of such problems and it is therefore an object of the present invention to provide a plasma processing system capable of producing a plasma over a large area in a uniform plasma density and of uniformly plasma-processing the surface of a wafer of a big diameter.